1. Field of the Invention
The present invention relates to a method for designing a semiconductor device including an oxide semiconductor.
The semiconductor device in this specification refers to all devices including a semiconductor element that can function by utilizing semiconductor characteristics; semiconductor circuits, electro-optic devices, display devices, and electronic devices are all semiconductor devices.
2. Description of the Related Art
A technique in which a transistor is formed using a semiconductor material such as amorphous silicon, polysilicon, or transferred single crystal silicon over a substrate having an insulating surface has been known. While a transistor including amorphous silicon has low field effect mobility, the transistor can be easily formed over a large-area glass substrate. On the other hand, while a transistor including polycrystalline silicon has relatively high field effect mobility, the transistor requires a crystallization step such as laser annealing and cannot necessarily be easily formed over a large-area glass substrate. Further, a transistor including single crystal silicon cannot necessarily be easily formed over a large-area substrate.
On the other hand, transistors including an oxide semiconductor as s semiconductor material have attracted attention. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and such a transistor is used as a switching element or the like of an image display device.
A transistor including an oxide semiconductor in a channel formation region (also referred to as a channel region) can have higher field-effect mobility than a transistor including amorphous silicon. An oxide semiconductor film can be easily formed over a large-area glass substrate by a sputtering method or the like and can be formed at a temperature of lower than or equal to 300° C.; therefore, a manufacturing process of the transistor including an oxide semiconductor is simpler than that of a transistor including polycrystalline silicon.
The transistors including an oxide semiconductor can be applied to, for example, transistors included in a switching element provided in a pixel portion or transistors included in a driver circuit portion in a display device. Note that a driver circuit of a display device includes, for example, a shift register circuit, a buffer circuit, or the like, and the shift register circuit and the buffer circuit further include a logic circuit. By applying the transistor including an oxide semiconductor to a logic circuit of a driver circuit, the driver circuit can be driven at higher speed than in the case where a transistor including amorphous silicon is applied.
In addition, the logic circuit can be formed with transistors all having the same conductivity type. By manufacturing a logic circuit using transistors all having the same conductivity type, a process can be simplified.
With the use of a glass substrate or a plastic substrate over which such a transistor including an oxide semiconductor is formed, provision of display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper has been considered.